1. Field of the Invention
The present invention relates to a continuous drying apparatus for porous web, suitable for use as a pressure drying apparatus applied to a dryer part in a paper machine or as a pressure drying apparatus for porous web other than paper (for example, a sheet drying apparatus).
2. Description of the Related Art
FIG. 4 is a schematic diagram of a conventional continuous drying apparatus for porous web (cited from Japanese Patent Publication No. HEI 1-56198). In this apparatus, as shown in FIG. 4, paper or other porous web 3 (such as a sheet) to be dried and a drying band (a dry felt or wire) 4 for supporting the porous web 3, together with an auxiliary wire 5, enter an air removal chamber 6 which is continuously exhausted of air 7 by a suction pump and are subjected to air removal processing to have a satisfactory heat conductivity, after which they pass through between two surface elements 1 and 8 impermeable to air.
In this case, the surface elements 1 and 8 embrace the porous web 3 over the entire width thereof in such a manner that the surface element 1 in contact with the porous web 3 is heated by a heating medium within a heating space 2. Furthermore, the surface element 8 in contact with the drying band 4 is cooled by a fluid flowing through a cooling space 11 so that water vaporized from the porous web 3 can be condensed within the drying band 4.
After the separation from the surface elements 1 and 8, the drying band 4 is further separated from the porous web 3 so that the condensed water within the drying band 4 is removed in a suction box 17.
Furthermore, a cooling space 11 is sealed against a hood 13 supported by a support beam 14 and against rolls 9 and 10 by means of appropriate seals 16a and 16b, respectively, with a cooling liquid flowing through the cooling space 11 being fed from a liquid supply port 12 and discharged from a liquid discharge port 15.
In this manner, the porous web 3 is embraced by the surface elements 1 and 8 and are externally heated and cooled to remove water (moisture) contained therewithin.
However, since the cooling liquid flowing through the cooling space 11 is sealed by the rolls 9 and 10 in such a conventional continuous drying apparatus for porous web, the cooling liquid will adhere to the surfaces of the rolls 9 and 10, resulting in a slip of the surface element 8 on rolls 9 and 10. In the case of high speed running in particular, this slip becomes significant resulting in a remarkable abrasion and extreme meandering of the drying band 4, obstructing the steady running.
Also, the support beam 14 seals between the hood 13 defining the cooling space 11 and the various members and constitutes a cooling space from the viewpoint of pressure resistant structure, so that the entire size is enlarged, taking a lot of labor and time for the replacement of the surface element 8 and the drying band 4. More specifically, since the surface element 8 and the drying band 4 have an endless structure, they must be slid in the direction orthogonal to the plane of FIG. 4 for the replacement.
Moreover, in the continuous drying apparatus for porous web of FIG. 4, a closed space is formed upstream of the cooling space 11 (more specifically, the region extending from the liquid supply port 12 up to the liquid discharge port 15) serving as a drying section, and the air removal chamber 6 is provided in the closed space to continuously discharge the air 7 therewithin by means of the suction pump, for executing the air removal processing. However, in order to increase the drying speed, the pressure within the closed space must be kept at about 1 Torr or below, so there is also a problem that the exhaust speed of the suction pump becomes too high.
Following is a test calculation of the required exhaust speed by way of example.
(1) Conditions:
a. drying band; width B.times.thickness t.times.void volume .PHI.=6.sup.m .times.0.003.sup.m .times.0.3 PA1 b. line speed u=1200 m/min PA1 c. degree of vacuum P.sub.1 =1 Torr PA1 where S: exhaust speed (m.sup.3 /min)
(2) Calculation of Exhaust Velocity EQU S=Bt.PHI.u.times.760/P=6.times.0.003.times.0.3.times.1200.times.760/l EQU =4.92.times.10.sup.3 m.sup.3 /min=4.92.times.10.sup.6 l/min (liter/min)
The suction pump can be an oil-sealed rotary vacuum pump or a mechanical booster pump from the conditions on the degree of vacuum. These characteristics are shown in FIGS. 5 and 6.
As is apparent from FIGS. 5 and 6, even in the condition (the condition (1) in both FIGS. 5 and 6) maximizing the required exhaust speed (l/min), it result in the vicinity of 1.times.10.sup.4 l/min at 1 Torr in the degree of vacuum (pressure P1), in other words, the result (4.92.times.10.sup.6 l/min) of the above calculation is about 100 times larger than these general specifications, which will be impractical.
Furthermore, FIG. 7 illustrates an influence of air (uncondensed gas) on the condensation heat transfer rate of vapor. As is clear from FIG. 7, accordingly as the air content is increased, diffusion of the vapor is blocked, resulting in a reduction in the condensation heat transfer rate. The range of the air content allowing a neglect of such an influence of the air is of the order of 0.002 kg (air)/kg (vapor), with the air content being 0.001 m.sup.3 (air)/m.sup.3 (vapor) in terms of volume ratio. That is, air partial pressure of 1 Torr or below corresponds to total pressure 1000 Torr of vapor (including air).